How Things Work: The Greenhouse Effect

The greenhouse effect is caused by sunlight that enters the Earth’s atmosphere, creating heat. Some sunlight reflects off the Earth into space, while some is trapped by the atmosphere due to gases like carbon dioxide. (credit: National Oceanic and Atmospheric Administration)

Greenhouse gases are named for their similarity to greenhouses, which trap the heat of sunlight using glass. The glass lets heat from sunlight in, but does not let it all escape, as the heat reflects downward off the glass. Greenhouse gases act like the glass by trapping heat within Earth’s atmosphere, raising the temperature of the Earth to a habitable level. (credit: Courtesy of Wikimedia Commons)

Imagine the Earth as a much colder planet. One could walk outside and watch his or her breath freeze upon contact with the open air. There would be no trees; even if there were, no animals would be playing in them. In fact, there would be very little of anything out there. Life, as it is today, would have actually ceased to exist. This would be our planet without the greenhouse effect.

The greenhouse effect is responsible for maintaining temperatures suitable for sustaining life. With it, the average temperature of Earth is around 60°F. Without it, the Earth would have an average temperature of about 0°F, resulting in a world as frigid as the one described above. This means that nearly 60 degrees Fahrenheit are attributed to this process. What exactly is going on here?
As the Earth’s main energy source, the sun emits energy in the form of visible light, and that energy is converted to heat at the surface of the Earth. According to the University Corporation of Atmospheric Research, the Earth must maintain a heat balance with outer space by emitting heat back into space in the form of infrared radiation that is invisible to the human eye. With no atmosphere and no absorbing “greenhouse gases,” this radiation would be lost to outer space and no heat would be retained. However, this is not the case, because greenhouse gases absorb this radiation and re-emit it in all directions — some continuing on to outer space and some back towards Earth. Essentially, it’s an atmospheric tennis match, and radiation is the ball.

One common myth you may have heard is that Earth’s temperature is “just right” because its orbit falls at a perfect distance from the sun. This is only part of the story, and as you may have guessed, it depends heavily on the constituents of our atmosphere and the strength of the greenhouse effect. For example, consider our two closest cosmic neighbors, Venus and Mars. The American Institute of Physics explains that Venus has a very thick and heavy atmosphere consisting mostly of carbon dioxide, sulfur dioxide, and water vapor — three greenhouse gases. As a result, its surface temperature is over 800°F, which is even hotter than Mercury’s surface temperature, even though Mercury is closer to the sun. In contrast, Mars has a very thin atmosphere and therefore a very weak greenhouse effect, resulting in a surface temperature of around –80°F.

The details of how greenhouse gases work are more complicated. Some greenhouse gases absorb radiation more than others; it all depends on the compatibility of the radiation and the gas. Infrared radiation emitted from the Earth has a spectrum of different wavelengths, a property that determines the types of interactions the light can have with different types of matter. (We perceive different wavelengths of visible light in the form of colors.) For example, if radiation of a given wavelength will increase the energy of a gas molecule, the gas molecule will absorb the radiation. In other words, a gas molecule will only absorb radiation if it is favorable, in terms of energy, to do so. The majority of Earth-emitted infrared radiation falls between 4 and 40 micrometers in wavelength, so any gas molecule that is able to achieve an increase in energy from absorbing any of these wavelengths is considered a greenhouse gas.

Currently, there is much scientific research being conducted to figure out to what extent humans are altering the Earth’s temperature by adding large amounts of greenhouse gases to our atmosphere. Despite the possible harsh implications of this so-called “enhanced greenhouse effect,” one must note how important a natural greenhouse effect here on Earth is to sustaining life as we know it.